Pull out height code.

The stuff that calculates the roof and building height from levels and whatnot needs to be easier to test.

Start by locating the code that calculates roof and building height from levels. Read how those calculations are currently used in building rendering, then identify a way to make them independently testable. Done means the height-calculation code is easier to test; the issue does not name files or tests.

Written by the indexing model from the issue text.